Add New Category box on Write->Link page is broken

Description

The AJAX category add box in edit-link-form.php needs to be updated. It's a cut-and-paste from the post form and needs to use link categories instead of post categories and lose all of the category hierarchy support since link categories are flat. See wp-admin/edit-link-form.php, wp-admin/js/link.js, and wp-admin/admin-ajax.php.